The field of the invention relates to satellites and more particularly to tracking satellites in nominally geostationary orbits about the earth.
The use of satellites for communications is well known. In principle, a satellite can be placed in a circular orbit in the equatorial plane at such a distance from the centre of the earth that the orbital period is equal to the rotational period of the earth. If the direction of revolution about the earth is the same as the direction of rotation of the earth, the satellite appears to remain motionless to an observer on the earth.
In general, the orbit cannot be strictly circular and in the equatorial plane even if a satellite could be placed initially in such a perfect orbit, external forces, such as the gravity of the moon and the sun, asymmetries in the earth""s gravitational field, and radiation pressures on the large photo-voltaic panel arrays of the satellite, all act to gradually change the orbital elements with time. Station-keeping manoeuvres may be employed to keep the apparent position of the satellite within defined limits.
Since the satellite moves in accordance with Kepler""s laws, any ellipticity of the orbit causes the satellite to move most quickly at perigee and most slowly at apogee. In general, the satellite""s orbital plane may be inclined to the equatorial plane so that, even if the satellite is in a strictly circular orbit, it appears to move primarily in a north-south direction with a small east-west component as viewed from the centre of the earth.
The beamwidth of the earth station antenna may be sufficiently wide that, even with the inevitable apparent motion of the satellite, the signal strength remains sufficiently constant that the earth station antenna may remain fixed.
Some applications may require an earth station antenna with greater gain. The antenna beamwidth is thereby reduced with the result that it may be necessary for the earth station antenna to track the apparent satellite motion to avoid large variations in the received signal strength. Secondly, it may become uneconomical or impossible to maintain the satellite in a geostationary orbit by station keeping manoeuvres even though the satellite is otherwise operational. In this case, the satellite service lifetime may be increased by including the capability of tracking the satellite apparent motion by the earth station antenna.
For a nominally geostationary satellite, the apparent motion of the satellite is relatively slow with a periodicity of approximately one sidereal day. In general, the received signal strength may be maximized at any time by executing a series of steps in azimuth and elevation so as to xe2x80x98climbxe2x80x99 to the position of maximum received signal strength. These step tracking techniques require many back-and-forth motions of the antenna in both azimuth and elevation that may result in excessive wear of the drive system. Since the result of each measurement is generally compared only with the immediately precedent measurement, the technique is not always reliable and may fail entirely in the presence of severe atmospheric scintillations or precipitation attenuation. Recovery from these conditions generally requires human intervention.
To increase the drive system reliability and reduce routine maintenance, it is desirable to reduce the number of motion requests which are required to peak the antenna. It is also desirable to determine the satellite direction with greater precision and to reduce the susceptibility of the antenna peaking process to scintillations and other fluctuations in the receive signal level.
For higher frequencies and many locations, the antenna cannot be peaked on the satellite during periods of significant precipitation attenuation. An antenna positioning system requires a technique which maintains alignment of the antenna with the satellite when normal antenna peaking is not possible due to precipitation attenuation.